The present invention pertains generally to ophthalmic surgical procedures which use pulsed laser beams to photoalter tissue of the cornea. More particularly, the present invention pertains to ophthalmic surgical procedures which effect the correction of optical aberrations identified by reference to diagnostically predetermined refractive power maps. The present invention is particularly, but not exclusively, useful for conducting an ophthalmic surgical procedure that is customized to correct the specific optical aberrations of a particular eye.
Optical aberrations of an eye generally manifest themselves as asymmetric distortions relative to the optical axis of the eye. Further, they are typically non-spherical in their geometry. Consequently, in order to successfully accomplish high-accuracy corrections of these optical aberrations, it is necessary to have detailed and accurate information about the optical aberrations. It is also necessary to have a surgical instrument that is capable of precisely and accurately performing the necessary surgical procedures.
Non-ultraviolet, ultrashort pulsed laser technology has now advanced to the point where pulsed laser beams can be produced that have pulses with durations measured in femptoseconds. For example, see U.S. Pat. No. 5,993,438 which issued to Juhasz et al. for an invention entitled xe2x80x9cIntrastromal Photorefractive Keratectomyxe2x80x9d (hereinafter the Juhasz Patent). Importantly, it is known that a device as disclosed in the Juhasz Patent is effective for performing ophthalmic surgical procedures with the precision that is required for high-accuracy corrections of optical aberrations.
As indicated above, in addition to having an effective surgical laser, it is also important to have detailed information about the optical aberrations that are to be corrected. Specifically, it is necessary to know the extent of the desired correction, and the location in the cornea of the eye where the correction can be made most effectively. For these purposes, it is necessary to precisely determine the refractive power of the cornea. It happens that wavefront analysis techniques, made possible by devices such as the well-known Hartmann-Shack type sensor, can be used to generate maps of corneal refractive power. These maps, or similar refractive power information provided by other means such as corneal topographs, can then be used by the ophthalmic surgeon to identify and locate the optical aberrations of a cornea that require correction.
In light of the above, it is an object of the present invention to provide a device, and a method for using the device, that can customize corneal corrections by effectively using a refractive power map as a tool for guiding an incising laser beam. It is another object of the present invention to provide a device, and a method for using the device, that effectively confines the photoalteration of corneal tissue to only the volume of tissue that is required to achieve the desired optical corrections for the cornea. Still another object of the present invention is to provide a device, and a method for using the device, that is easy to implement, simple to use, and comparatively cost effective.
In accordance with the present invention, a method for customizing corneal corrections includes first obtaining information about the optical aberrations of the particular eye that is to be corrected. Typically, this information will be in the form of a refractive power map, such as a corneal topograph. As is well known, refractive power maps of this type can be generated using wavefront analysis techniques that will show the amount of correction required in each particular area of the cornea.
With the refractive power map in mind, an imaginary reference field (grid) is superposed on the anterior surface of the cornea of the eye. The purpose of this imaginary reference field (grid) is really two-fold. First, the imaginary reference field (grid) is used to divide the corneal surface into a plurality of individual reference areas that can be identified with corresponding portions of the refractive power map. Second, the reference areas provide targets for control in aiming, or directing, an incising laser beam toward the stromal tissue that is to be photoaltered. Further, by projecting each of the reference areas from the anterior surface through the cornea, the reference areas can be extended to define respective reference volumes. As contemplated by the present invention, the reference area will be generally square and quite small, with sides on the order of about ten microns in length. The reference volume will then extend through the cornea between the anterior and posterior surfaces of the cornea.
In operation, the incising laser beam is directed along a beam path and through a selected reference area to a focal spot. The stromal tissue at the focal spot is then photoaltered. It is known that lasers may be used for plasma mediated tissue ablation (generally superficial tissue) and for plasma mediated tissue disruption (generally internal bulk tissue). With this in mind, the term photoalteration is used in the context of the present invention to indicate an operation wherein there may be either plasma mediated tissue ablation or plasma mediated tissue disruption. Preferably, for the present invention, the incising laser beam is a non-ultraviolet ultrashort-pulsed laser beam having a plurality of pulses that each have a duration greater than approximately ten femtoseconds.
While the incising laser beam is activated, its focal spot is selectively moved along the beam path to photoalter stromal tissue through a predetermined length of the beam path. The incising laser beam is then redirected through another reference area and the process of photoalteration is repeated. In particular, the sequence for directing the incising laser beam through individually selected reference areas, and the extent of stromal tissue photoalteration while the incising laser beam is so directed, can be varied as required. Specifically, as indicated above, the amount of photoalteration will be determined by the refractive power map. On the other hand, the sequence of reference areas that is followed during a customized procedure will depend on the particular objectives of the procedure.
As a complementary procedure, the present invention envisions a LASIK type procedure wherein a flap is cut into the cornea to establish extracorporeal access to the tissue that is to be photoaltered. Once access has been achieved, the photoalteration is accomplished and the residual fragments of the photoaltered tissue are mechanically removed from the cornea. In another complementary procedure, it is envisioned that the photoalteration of intrastromal tissue will result in the creation of an isolated lenticle of intrastromal tissue. For this procedure, once the lenticle of tissue has been created it can be mechanically removed from the cornea.
In all of the customized procedures contemplated by the present invention, photoalteration of stromal tissue will be accomplished in accordance with the dictates of a refractive power map. Thus, regardless whether the procedure creates a lenticle or generates residual fragments that require subsequent removal from the cornea, or whether photoalteration alone is sufficient, the result is a customized reconfiguration of the cornea that will correct the specific optical aberrations of a particular eye.